Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery of choice for general purposes, such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multi-cell structure. Each cell comprises a set of vertical positive and negative plates formed of lead-acid alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged comprises lead dioxide, which is the positive acting material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte, based on sulfuric acid, is interposed between the positive and nagative plates.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce light-weight lead-acid batteries, especially in the aircraft, electric car and vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density.
Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connected electrodes or cells. The bipolar plates must be impervious to elctrolyte and be electrically conductive to provide a serial connection between electrodes.
U.S. Pat. Nos. 4,275,130, 4,353,969; 4,405,697; 4,539,268; 4,507,372; 4,542,082; 4,510,219; and 4,547,443 relate to various aspects of lead-acid batteries. Certain of these patents discuss various aspects of bipolar plates.
Attempts have been made to improve the conductivity and strength of bipolar plates. Such attempts include the use of conductive carbon particles or filaments such as carbon, graphite or metal in a resin binder. However, carbonaceous materials are oxidized in the agressive electrochemical environment of the positive plates in the lead-acid cell to acetic acid, which in turn reacts with the lead ion to form lead acetate, which is soluble in sulfuric acid. Thus, the active material is gradually depleted from the paste and ties up the lead as a salt which does not contribute to the porduction or storage of electricity.
The metals fare no better; most metals are not capable of withstanding the high potential and strong acid environment present at the positive plates of a lead-acid battery. While some metals, such as platinum, are electrochemically stable, their prohibitive cost prevents their use in high volume commercial applications of the lead-acid battery.